In a randomized controlled trial, with 3, 6-month and 1-year follow-up, this application seeks to assess the effectiveness of using an online peer support community intervention to increase MAT initiation and sustainment among participants with opioid use disorder (OUD) who are recruited online. On average, 130 people die in the United States each day from opioid overdoses, making the opioid crisis the country's top public health concern (1,2). Although effective FDA-approved medication-assisted therapy (MAT) is available for patients with opioid use disorder (OUD), including buprenorphine and injectable naltrexone, only a small fraction of those who would benefit from these medications use them. There are a number of reasons for low MAT use, including lack of insurance; knowledge, among both providers and patients; MAT-related stigma; and social norms (e.g., people would be more willing to use MAT if their peers were also using MAT). Innovative methods, especially those that address multiple levels (e.g., the individual, culture, and society/community) are needed to increase MAT use among individuals with OUD. Online peer-led support interventions might be particularly effective at increasing MAT uptake because they can leverage peers to widely and rapidly scale changes in social norms (e.g., interest in using MAT) throughout people's natural, real-world, virtual environments. For example, the Harnessing Online Peer Education (HOPE) intervention, an online peer support community intervention designed to reduce stigma and increase health behavior change, has been shown to be effective at changing health behaviors among stigmatized populations, such as for HIV. Specific to opioid-related issues, our team recently found the HOPE intervention to be a feasible and acceptable method of reducing risk for prescription opioid misuse among chronic pain patients on opioid therapy. However, no research has studied whether online support interventions, such as HOPE, might be effective in increasing MAT requests, uptake, and sustainment among OUD patients. It is therefore critical to evaluate these social normative/behavioral technology methods to reduce the increasing rate of opioid overdose. In this study, we seek to explore whether and how the HOPE online support intervention might be adapted to increase MAT initiation and sustainment among participants with OUD, assess the effectiveness of using the intervention to increase MAT use among OUD participants recruited online who are not using MAT, and using an implementation science approach (based on constructs from the Consolidated Framework for Implementation Research, CFIR, determine the relationship between social network dynamics (e.g., network size), topics discussed on the online community, and behavior change.